1. Field of the Invention
The present invention relates to a process and to an apparatus for the purification of crude diphenyl-p-phenylenediamine (PPD) and to the use of purified DPPD.
2. Description of the Background
DPPD is used on a large industrial scale as a stabilizer, inter alia, in the reaction of potassium methacrylate with 3-chloropropyltrimethoxysilane. In this application, the use of commercially available DPPD can impair product quality, in particular the color number of the product. Unfortunately, in the past, industrial quantities of DPPD have only been available on the market in a technical quality.
Crude DPPD, that is, commercially available DPPD or technically pure DPPD. has, as a rule, a purity in the range around 85% by weight, the remainder of about 15% by weight essentially consisting of unconverted or incompletely converted starting materials such as, for example, diphenylamine, and of inorganic components such as iron and chlorine, for example in the form of iron chloride. The commercial material is gray and is often in the form of a powder or in the form of flakes. In contrast thereto, pure DPPD is white and crystalline.
Several methods for the purification of DPPD are described in the literature. It is possible to crystallize DPPD from chlorinated hydrocarbons or carbon tetrachloride (Beilstein E III 13, 4.sup.th Auflage 1973, pages 115-118, The Merk Index, 9.sup.th Edition 1976, page 445). The disadvantage of these processes is, however, the use of chlorinated solvents which are toxic and pollute the environment.
Another purification method is solids distillation. This method can be applied successfully in the laboratory. For the purification of crude DPPD on an industrial scale, however, solids distillation is not a suitable process, since it is too expensive in engineering terms. A need therefore continues to exist for a method of inexpensively and effectively purifying DPPD.